Anodic electrophoretic coating method

ABSTRACT

A method for anodic electro-dip lacquer coating, wherein coating medium which is consumed in an anodic electro-dip bath is compensated for by an under-neutralised anodic replenishment material, wherein the replenishment material comprises 
     A) a pigment-free aqueous binder vehicle component with a solids content of 40 to 70% by weight, an MEQ value of 15 to 40 and a content of organic solvent of ≦0.5% by weight, and 
     B) a pigment-containing aqueous paste resin component with a solids content of 60 to 75% by weight, an MEQ value of 5 to 15 and a content of organic solvent of ≦1.0 % by weight, 
     wherein A) and B) are present in a ratio by weight of 1:1 to 4:1 and the mixture of A) and B) has a solids content of 45 to 73% by weight, a solvent content of ≦0.75% by weight and an MEQ value which is 50 to 70% lower than the MEQ value of the electro-dip bath.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing an anodicelectro-dip lacquer coating (ADL) using an electro-dip lacquer coatingbath (ADL bath) which is low in solvents or free from solvents, whereinit is not necessary to perform electrodialysis in the EDL bath in orderto maintain the bath and coating parameters. Therefore, it is also notnecessary to discard ultrafiltrate on a regular basis.

The principle of anodic electro-dip lacquer coating (ADL) is describedin the literature and has been proven in practice. Even after theintroduction of cathodic electro-dip lacquer coating (CDL), anodicelectro-dip coating is still a widely used coating method, particularlyfor the coating of industrial products. This is due firstly to the largenumber of existing anodic coating installations, and secondly to thegood quality of anodic coating materials which is achieved nowadays.Moreover, certain materials, such as aluminium for example, can becoated more advantageously using anodic rather than cathodic electro-diplacquer compositions. In anodic electro-dip lacquer coating a workpiecehaving an electrically conducting surface comprising a metal or anelectrically conducting plastics material or comprising a substratewhich is provided with an electrically conducting coating is placed inan aqueous ADL bath and is connected as an anode to a source of directcurrent.

The ADL bath consists of an aqueous dispersion, e.g. a suspension oremulsion, or of an aqueous solution of one or more binder vehicles whichhave been made at least partially dispersible or soluble in water bysalt formation with organic or inorganic neutralizing agents, and ofpigments, extenders, additives and other adjuvant substances which aredispersed therein.

When a DC electric current is applied, the polymer particles of theaqueous dispersion of the ADL bath migrate to the anode and react againthere with the ions formed during the electrolysis of water, whichproceeds simultaneously, to form a water-insoluble polymer whichcoagulates from the aqueous phase and is deposited, with the additivesdispersed therein, as a lacquer film on the anode (Metalloberfläche31(1977) 10, pages 455 to 459).

The usual ADL baths are operated continuously, i.e. the substratesdescribed above are immersed and coated in an electro-dip lacquer tankfilled with coating medium. Solids are thereby dragged out of the ADLbath and neutralizing agent is released in the ADL bath at the sametime. In order to maintain the coating parameters and the quality of thecoating constant, it is necessary to add replenishment material with anincreased solids content to the ADL bath in order to compensate for thedragged-out solids and in order to compensate for the neutralizing agentreleased in the ADL bath so as to maintain the desired MEQ value.

In principle, there are two compensating procedures which are employedin order to compensate for the solids dragged out of the ADL bath and tocompensate for the neutralizing agent released. The added replenishmentmaterial with an increased solids content is neutralized to a lesserextent than the ADL bath, and the neutralizing agent which is releasedis required for the dispersion and homogenisation of the replenishmentmaterial in the ADL bath and is thereby consumed. Compensation can alsobe effected using completely neutralized replenishment material.However, the equipment costs are then increased, since the neutralizingagent which is released has to be removed by means of (electro)dialysis(Glasurit-Handbuch 1984, page 377 and Willibald Machu“Elektrotauchlackierung”, Verlag Chemie GmbH Weinheim/BergstraBe, 1974,page 166). The neutralizing agent which is released during the coatingoperation can also be removed by discarding ultrafiltrate on a regularbasis. When compensation for the neutralizing agent which is releasedduring the coating operation is effected by a replenishment materialwith a lesser degree of neutralization, the latter requires a highcontent, of up to about 15% by weight, of organic solvents, since it isotherwise unstable and its viscosity is too high, and it cannot beincorporated in the coating material, which can contain more than 90%water. Coating media of this type are described, for example, in DE-A-3247 756.

In Farbe und Lack 103, Number 6/97, page 26, there is a reference to anew, environmentally friendly anodic single-component system (1 Csystem) for electro-dip lacquer coating. However, the replenishmentpaste, which is the form in which the replenishment material issupplied, still always contains 6% of organic solvents, and the bathstill always contains 0.5% of organic solvents when it is in operation.

However, high solvent contents are undesirable on account of thepollution of outgoing air and waste water, wherein the total usage ofsubstances is calculated based on legal regulations. In order to removethe neutralizing agent which is released during the coating operation,the cathodes in the ADL bath can also be accommodated in re-flushabledialysis cells (electrodialysis) and the neutralizing agent which isformed there can be discarded, or the coating material can be subjected,continuously or discontinuously, to an ultrafiltration step, with theultrafiltrate which is thus produced being discarded on a regular basis.Electrodialysis devices of this type are not used in most ADL baths, onaccount of increased capital costs and higher maintenance and inspectioncosts. Moreover, regularly discarding ultrafiltrate or dialysate resultsin an increased cost of waste water processing and is thereforeundesirable. The make-up of electro-dip lacquer baths with completelyneutralized material, consisting of one or two components, is known fromthe literature (Glasurit-Handbuch 1984, page 377) and is described thereusing cathodic electro-dip lacquer coating as an example. As mentionedabove, however, the use of electrodialysis and the discarding ofdialysate is absolutely necessary in the procedure described there.

The object of the present invention was therefore to provide a method ofproducing an aqueous coating composition, which is low in solvents orfree from solvents, for anodic electro-dip lacquer coating, for which,when it is used for the coating of conductive substrates in an ADL bath,it is not necessary to remove neutralizing agent which is releasedduring the coating operation by an electrodialysis device in order tomaintain the bath and coating parameters, and a considerable amount ofultrafiltrate does not have to be discarded on a regular basis.

Surprisingly, this object has been achieved by the use of an anodicreplenishment material consisting of a pigment-free aqueous bindervehicle component and a pigment-containing aqueous paste resin componentin order to compensate for the coating material consumed duringelectro-dip lacquer coating and for the neutralizing agent which isreleased at the same time, which anodic replenishment material isunder-neutralized to an extent such that when added to the ADL bath itcompensates for the neutralizing agent released there, and whichnevertheless only contains small amounts of organic solvents.

SUMMARY OF THE INVENTION

Therefore, the present invention firstly relates to a method for anodicelectro-dip lacquer coating, wherein coating medium which is consumed inan anodic electro-dip bath is compensated for by an under-neutralizedanodic replenishment material, which is characterised in that thereplenishment material comprises

A) a pigment-free aqueous binder vehicle component with a solids contentof 40 to 70% by weight, an MEQ value of 15 to 40 and a content oforganic solvent of ≦0.5% by weight, and

B) a pigment-containing aqueous paste resin component with a solidscontent of 60 to 75% by weight, an MEQ value of 5 to 15 and a content oforganic solvent of ≦1.0% by weight,

wherein A) and B) are present in a ratio by weight of 1:1 to 4:1 and themixture of A) and B) has a solids content of 45 to 73% by weight, asolvent content of ≦0.75% by weight and an MEQ value which is 50 to 70%lower than the MEQ value of the electro-dip bath.

DETAILED DESCRIPTION OF THE INVENTION

The solids content of components A) and B) can be measured, according toDIN EN ISO 3251 for example, for 30 minutes at 180° C. The solidscontent of component A) is preferably 45 to 65% by weight. The solidscontent of component B) is preferably 60 to 73% by weight.

The MEQ value of component A) is preferably 20 to 35, and the MEQ valueof component B) is preferably 5 to 10. The MEQ value is a measure of thecontent of neutralizing agent in an aqueous lacquer. It is defined asthe amount of milliequivalents of neutralizing agent with respect to 100g solids.

The content of organic solvent of component A) is preferably ≦0 4% byweight, and that of component B) is preferably ≦0.5% by weight.

The mixture ratio of component (A) to component (B) ranges from 1:1 to4:1, preferably from 2:1 to 3.5:1 with respect to the weight of theaqueous component concerned.

The mixture has a solids content of 45 to 73% by weight, a solventcontent of 0.75% by weight at most, and an MEQ value which is 50 to 70%less, preferably 60 to 70% less, than the MEQ value of the ADL bath inits state in which it is capable of coating.

Component (A) contains the binder vehicle or binder vehicles of theaqueous coating medium and also optionally contains a biocidalcomponent, and contains crosslinking agents if necessary, and alsooptionally contains emulsifiers, film-forming agents, other additivessuch as neutral resins and customary lacquer additives such as lightstabilisers and optical brighteners for example.

Component (B) contains one or more paste resins, pigments and/orextenders, optionally contains a biocidal component and containscrosslinking agents if necessary, and also optionally containsfilm-forming agents and customary lacquer additives as well as otheradditives, such as those which may be contained in component (A) forexample.

Binder vehicle systems which are suitable for use as binder vehicles ofcomponent (A) comprise all those with an acid number of 20 to 150,preferably 20 to 120, and a hydroxyl number of 20 to 150, preferably 60to 120, such as those which are known for aqueous coating systems,particularly for anodic electro-dip lacquer coatings.

Examples thereof include polyester, polyacrylate and polyurethaneresins; modified polyester or polyurethane resins, such as alkyd resinsfor example, urethanised polyester resins or acrylated polyester orpolyurethane resins, as well as mixtures of these resins. Polyesterresins are preferred.

Examples of suitable polyester resins in component (A) includepolyesters which contain carboxyl groups and hydroxyl groups and whichhave an acid number of 20 to 150 and a hydroxyl number of 20 to 150.These are produced by methods known to one skilled in the art, namely bythe reaction of polyhydric alcohols with polyvalent carboxylic acids orcarboxylic acid anhydrides, and optionally with aromatic and/oraliphatic monocarboxylic acids also. The necessary content of hydroxylgroups is obtained in the manner known in the art by suitably selectingthe type and quantitative ratios of the starting materials. Carboxylgroups can be introduced, for example, by forming a semi-ester from apolyester resin, which has been produced previously and which containshydroxyl groups, and acid anhydrides. Carboxyl groups can also beincorporated, for example, by the use of hydroxycarboxylic acids inconjunction during the condensation polymerisation reaction.

The dicarboxylic acids and the polyols can be aliphatic or aromaticdicarboxylic acids and polyols.

Examples of low molecular weight polyols which are used for theproduction of the polyesters include low molecular weight polyols, e.g.diols such as alkylene glycols, for example ethylene glycol, butyleneglycol, hexanediol, hydrogenated bisphenol A and2,2-butyl-ethyl-propanediol, neopentyl glycol and/or other glycols suchas dimethylolcyclohexane. Components of higher functionality or mixturesof mono-functional OH components with components of higher functionalitycan also be used, such as trimethylolpropane, pentaerythritol, glycerolor hexanetriol; polyethers which are condensates of glycols withalkylene oxides; or monoethers of glycols such as these, e.g. diethyleneglycol monoethyl ether or tripropylene glycol monomethyl ether.

The acid component of the polyester preferably consists of low molecularweight dicarboxylic acids or anhydrides thereof which contain 2 to 18carbon atoms in their molecule.

Examples of suitable acids include phthalic acid, isophthalic acid,terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid,adipic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid,glutaric acid, succinic acid, itaconic acid and/or1,4-cyclohexane-dicarboxylic acid. Instead of these acids, the methylesters or anhydrides thereof can also be used, provided that they exist.In order to obtain branched polyesters, it is also possible to addproportions of carboxylic acids of higher functionality, such astri-functional carboxylic acids, trimellitic acid, malic acid, aconiticacid or bishydroxyethyl taurine, as well as dimethylolpropionic acid,dimethylolbutyric acid or bisanhydrides. Polycarboxylic acids which donot form cyclic anhydrides are preferred.

The polyester resins can also be modified, for example by theincorporation of unsaturated compounds or of compounds which containisocyanate groups, or by partial or graft polymerisation withethylenically unsaturated compounds.

Examples of polyesters which are preferred in component (A) includepolyesters which contain carboxyl groups, and which have an acid numberof 20 to 120 and a hydroxyl number of 20 to 150, preferably 60 to 120.For example, these can be the reaction products of di- and/or polyhydricaliphatic or cycloaliphatic saturated alcohols with aliphatic,cycloaliphatic and/or monocyclic aromatic di- or polybasicpolycarboxylic acids and can optionally be the reaction products oflinear or branched, saturated or unsaturated aliphatic and/orcycloaliphatic C₃ to C₂₀ monoalcohols or monocarboxylic acids. Thequantitative ratios of the starting materials are calculated from themolar ratios which result in the desired acid numbers and hydroxylnumbers of the resin. The selection of the individual starting materialstaking into account the intended use of the product is known to oneskilled in the art.

The number average molecular weight Mn, as measured using polystyrene asthe calibration substance, ranges from 1000 to 6000, and is preferably2000 to 4000. Oil-free polyesters which contain carboxyl groups areparticularly preferred, such as those described in DE-A-32 47 756 forexample.

These polyesters preferably contain 0.3 to 3.0, most preferably 0.5 to2.5 milliequivalents per gram of resin of aliphatic, cycloaliphaticand/or monocyclic aromatic dicarboxylic acids, which are incorporated bycondensation. When using cyclic carboxylic acids, 0.8 to 2.0, preferably0.9 to 1.8, most preferably 1.1 to 1.5 millimoles of these acids areadvantageously bonded to the polyester via one carboxyl group only. Tri-and/or polybasic polycarboxylic acids, most preferably tri- and/ortetrabasic acids, are preferably used as polycarboxylic acids. Thepolyesters are produced in the manner known in the art by thecondensation polymerization of the starting materials, a step-wiseprocedure preferably being employed to prevent the occurrence ofturbidity and gel formation.

The esterification of what are preferably aromatic and cycloaliphaticdicarboxylic acids which are not capable of forming an intramolecularanhydride is preferably effected with dialcohols which either containsecondary OH groups or which contain primary OH groups which aresterically hindered due to substitution, a polyester which contains OHgroups being formed by the use of excess alcohol. The alcoholspreferably contain 2 to 21, most preferably 4 to 8 C atoms. Thedicarboxylic acids preferably contain 5 to 10 C atoms, most preferably 6C atoms.

Examples thereof include isophthalic acid, terephthalic acid, 1,3- and1,4-cyclohexane-dicarboxylic acid, or alkyl-substituted dicarboxylicacids comprising butyl isophthalic acid. Isophthalic acid isparticularly preferred. In order to obtain a branched product, acorresponding amount of a tricarboxylic acid such as trimelliticanhydride can be incorporated by condensation in the resin molecule inplace of a proportion of the dicarboxylic acid. On the other hand,dimethyl esters, such as, dimethyl terephthalate or1,4-cyclohexane-dicarboxylic acid dimethyl ester can also be introducedinto the polyester by transesterification, optionally, in the presenceof transesterification catalysts.

The dialcohols which are preferably used are neopentyl glycol,hydroxypivalic acid neopentyl glycol ester, 2,5-hexanediol,1,4-bis(hydroxymethyl)cyclohexane,1,1-isopyrilidine-bis-(p-phenoxy)-2-propanol and2,2,4-trimethylpentanediol-1,3, as well as mixtures thereof.

Glycidyl esters of α-branched fatty acids, such as, versatic acid, canbe used as the alcohol, because the fatty acid is incorporated in themolecule so that it is stable to hydrolysis. In special cases it is alsopossible to use epoxy resins, the epoxy groups of which have beenreacted with monoalcohols.

It is possible to use proportions of polyols comprising more than two OHgroups, such as, trimethylolpropane or pentaerythritol, in order toobtain suitable OH numbers and viscosities. The same applies to a slightmodification, to impart elasticity, with long chain dialcohols such as1,6-hexanediol or with aliphatic dicarboxylic acids such as adipic acid.

This esterification (the first step) is conducted in the known manner,namely azeotropically or in the melt at an elevated temperature (above190° C.), and results in a clear product with an acid number of 0 to 50,preferably 5 to 25, and a viscosity of 200 to 3000 mPas at 25° C. asmeasured in a 75% solution in butyl glycol.

To impart solubility in the aqueous alkaline medium, carboxyl groupshave to be introduced in addition into the polyesters which contain OHgroups. For this purpose, a reaction is effected at temperatures below190° C. with an aromatic or cycloaliphatic dicarboxylic acid which haspreferably been produced, by defunctionalisation with a long chain,aliphatic hydrophobic monoalcohol, from a polycarboxylic acid comprisingthree or four carboxyl groups, such as, trimesic acid, hemellitic acid,prehnitic acid or mellophanic acid for example. This method isparticularly simple when anhydride-containing compounds are used, suchas, trimellitic anhydride, pyromellitic anhydride or correspondinghydrogenated ring systems, and when cyclopentane-tetracarboxylicanhydride or pyrazine-tetracarboxylic anhydride is used.

The polycarboxylic acids can be reacted stoichiometrically, by a two-potmethod for example, with an amount of monoalcohol such that adicarboxylic acid is obtained which is subsequently added to thepolyester which contains OH groups at temperatures of about 150 to 190°C.

In practice, a single-pot method of producing the polyesters whichcontain carboxyl groups has proved useful in which approximately thestoichiometric amounts of monoalcohol and trimellitic anhydride areadded in the given sequence to the polyester, which contains OH groups,in the first step.

Examples of monoalcohols which can be used include linear and/orbranched, saturated and/or unsaturated, primary, secondary and/ortertiary alcohols, preferably primary and/or secondary alcohols.Mixtures of these alcohols can also be used, particularly isomericmixtures. Aliphatic C6 to C18 monoalcohols are preferred, as are benzylalcohol and alkyl-substituted products thereof. Branched chain C8 to C13iso-monoalcohols are particularly preferred. Semi-esters which areparticularly stable towards hydrolysis are obtained by the use of(α-branched monoalcohols or secondary monoalcohols, such as,cyclohexanol or secondary methyl octyl alcohol. It is ensured by thesynthesis of the resins that any cleavage products which are possiblyformed by hydrolysis (monoalcohols and monoesters of trimellitic acid)are electrophoretically deposited with the film without problems.

Carboxyl groups can also be incorporated, for example, by the use inconjunction during the condensation polymerisation reaction ofhydroxycarboxylic acids, such as, dimethylolpropionic acid for example,the free carboxyl group of which does not generally take part in thecondensation polymerisation reaction on account of steric hindrance, sothat this acid is incorporated exclusively via hydroxyl groups.

The molar ratios of the overall formulation for the production of thepolyesters are selected so that a viscosity is obtained which issuitable for the purpose of use in question. This viscosity, forexample, is about 200 to 3000, preferably 250 to 2000 and mostpreferably 300 to 1500 mPas, as measured in a 50% solution in butylglycol at 25° C. The viscosity can also be adjusted, as can themolecular weight, by admixture with resins of higher or lower viscosityor with resins of lower or higher molecular weight, respectively. Theupper limit of the acid number is preferably less than 100, mostpreferably less than 60; the lower limit of the acid number ispreferably greater than 35, most preferably greater than 40. Thepolyester which contains carboxyl groups contains at least one,preferably at least two carboxyl groups per molecule in order to achievesolubility in water by salt formation with a low molecular weight base.If the acid number is too low, the solubility is too low. If the acidnumber is too high, the high degree of neutralization gives rise to anincreased extent of electrolysis in the ADL bath, which can result insurface defects. The excess of alcohol which is selected results in ahydroxyl number of about 20 to 150, preferably 60 to 120, in thefinished resin. Resins are preferred which have a relatively highhydroxyl number and a low acid number.

Condensation polymerization is effected, azeotropically for example, orin the melt for example, at reaction temperatures between 160 and 240°C., preferably between 160 and 210° C. After the desired final resinvalues have been reached as regards viscosity and acid number, the batchis cooled to a temperature such that a product is formed which has aviscosity which ensures that water can be incorporated. In practice,this means that the melt viscosity which is reached should not exceed40,000 mPa.s. This can be achieved by cooling to a suitable temperature.Unless the reaction is conducted under pressure, this temperature isabout 100° C. at most.

To convert it into an aqueous solution or dispersion, the product of thecondensation polymerization is neutralized. For this purpose, theneutralizing agent can be added to the condensation polymerization resinbefore or during the addition of water, or can also be contained in thewater in which the polymerization resin is dispersed. High-speedagitator disc units, rotor-stator mixers or high-pressure homogenizersare used in the course of this procedure, for example. Organic solventscan optionally be removed by distillation during or after the conversioninto an aqueous solution or dispersion.

Neutralizing agents which are suitable for this purpose includecustomary bases, such as ammonia for example; primary, secondary andtertiary amines, such as, diethylamine, triethylamine or morpholine;alkanolamines, such as diisopropanolamine, dimethylaminoethanol,triisopropanolamine or dimethylamino-2-methylpropanol; quarternaryammonium hydroxides, or optionally small amounts of alkylene polyaminesalso, such as, ethylenediamine. Mixture of neutralizing agents of thistype can also be used.

The stability of the aqueous dispersion can be influenced by the choiceof neutralizing agent. The amount of neutralizing agent is selected sothat the MEQ value of the mixture of component (A) and component (B) is50 to 70% lower than the MEQ value of the ADL bath.

Example of suitable polyacrylate resins in component (A) includecopolymers which contain carboxyl groups and/or sulphonic acid groupsand which have an acid number of 20 to 150 and a number averagemolecular weight Mn of 1000 to 10,000.

The latter are produced by customary methods, namely by thecopolymerization of olefinically unsaturated monomers, wherein monomerswhich comprise acid groups are copolymerized with other monomers.Monomers which comprise acid groups are used in conjunction for thepurpose of incorporating carboxyl and/or sulphonic acid groups in thecopolymers. Due to their hydrophilic character, these groups ensure thatthe copolymers are soluble or dispersible in water, particularly afterwhat is at least a partial neutralization of the acid groups.

In principle, all olefinically unsaturated, polymerisable compoundswhich contain at least one carboxyl and/or sulphonic group are suitableas monomers which comprise acid groups, such as, olefinicallyunsaturated mono- or dicarboxylic acids, e.g., acrylic acid, methacrylicacid, crotonic acid, fumaric acid, maleic acid or itaconic acid, orolefinically unsaturated compounds which contain semi-esters of fumaricacid, maleic acid and itaconic acid or sulphonic acid groups, such as2-acrylamido-2-methylpropanesulphonic acid, for example, or any mixturesof olefinically unsaturated acids of this type. Acrylic acid andmethacrylic acid are particularly preferred.

In order to achieve the desired application technology properties in thefinished lacquer, the copolymers may contain other functional monomers,with which crosslinking reactions can be effected, for example, inaddition to the monomers comprising acid groups. These copolymers may beself-crosslinking or may be externally crosslinkable with othercomponents which are additionally introduced into the lacquer.

Examples of functional groups of this type include hydroxy, amino,amido, keto, aldehyde, lactam, lactone, isocyanate, epoxy and silanegroups. Olefinically unsaturated monomers are known which comprisefunctional groupings of this type. Hydroxy and epoxy groups areparticularly preferred. Furthermore, any non-functional olefinicallyunsaturated monomers can in principle be used in conjunction during theproduction of the copolymers.

Examples of suitable non-functional monomers include esters of acrylicand methacrylic acid, the alcohol components of which contain 1 to 18 Catoms, aromatic vinyl compounds, vinyl esters of aliphaticmonocarboxylic acids, acrylonitrile and methacrylonitrile.

The copolymers can be produced by polymerization by customary methods.Production of the copolymers is preferably conducted in an organicsolution. It is possible to use continuous or batch methods ofpolymerization.

Suitable solvents include aromatic compounds, esters, ethers andketones. Glycol ethers are preferably used.

Copolymerization is generally conducted at temperatures between 80 and180° C. using customary initiators, such as aliphatic azo compounds orperoxides for example. Customary regulators can be used for regulatingthe molecular weight of the polymers. After polymerization is complete,the copolymers can be neutralized as described for the condensationpolymerization resins and can be converted into an aqueous solution ordispersion, whereupon the organic solvent can optionally be removed bydistillation.

Examples of polyurethane resins which are suitable in component (A)include anionic polyurethane resins which contain carboxyl, sulphonicacid and/or phosphonic acid groups which are present in salt form. Theseare produced in the manner known in the art from polyols,polyisocyanates and optionally from chain extension agents.

The polyurethane resins can be produced either in bulk or in organicsolvents which are not capable of reacting with isocyanates. They areconverted into the aqueous phase by neutralization of their acid groups,as described for condensation polymerization resins. It is advisable inmany cases to produce the polyurethane resins in stages.

Thus it is possible, for example, first of all to produce a prepolymercomprising acid groups and terminal isocyanate groups in organicsolvents, which prepolymer, after neutralization of the acid groups withtertiary amines, is subjected to a chain extension procedure and isconverted into the aqueous phase, whereupon the organic solvents can beremoved by distillation.

The polyols which are used for the production of the prepolymer can beof low and/or high molecular weight and may also contain anionic groups.

Low molecular weight polyols preferably have a number average molecularweight Mn of 60 to 400 and may contain aliphatic, alicyclic or aromaticgroups. They can be used as up to 30% by weight of the total polyolconstituents.

Examples of suitable low molecular weight polyols include diols, triolsand polyols, such as, ethylene glycol, diethylene glycol, triethyleneglycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butyleneglycol, 1,6-hexanediol, trimethylol-propane, castor oil or hydrogenatedcastor oil, pentaerythritol, 1,2-cyclohexanediol,1,4-cyclohexanedimethanol, bisphenol A, bisphenol F, neopentyl glycol,hydroxy-pivalic acid neopentyl glycol ester, hydroxyethylated bisphenolA, hydrogenated bisphenol A and mixtures of these polyols.

High molecular weight polyols consist of linear or branched polyols withan OH number of 30 to 150. They can be used as up to 97% by weight ofthe total polyol constituents. They are preferably saturated orunsaturated polyester- and/or polyether diols and/or polycarbonate diolswith a molecular weight Mn of 400 to 5000, or mixtures thereof.

Examples of suitable linear or branched polyether diols includepoly(oxyethylene) glycols, poly(oxypropylene) glycols and/orpoly(oxybutylene) glycols.

Polyesters are preferred, and are produced in the known manner by theesterification of dicarboxylic acids or anhydrides thereof with diols.In order to produce branched polyesters, small amounts of polyols orpolycarboxylic acids of higher finctionality can also be used.

The groups which are capable of forming anions may originate from thepolyester or can be introduced into the prepolymer by the use inconjunction of compounds which contain two active H groups which reactwith isocyanate groups and at least one group which is capable offorming anions. Suitable groups which react with isocyanate groupsinclude hydroxyl groups in particular, as well as primary and/orsecondary amino groups. Examples of groups which are capable of forminganions include carboxyl, sulphonic acid and/or phosphonic acid groups.Examples of compounds which contain groups such as these includedihydroxycarboxylic acids, such as, dihydroxypropionic acid,dihydroxybutyric acid, dihydroxysuccinic acid, diaminobenzoic acid andpreferably α,α-dimethylolalkanoic acids, such as, dimethylolpropionicacid for example.

Suitable polyisocyanates include aliphatic, cycloaliphatic and/oraromatic polyisocyanates which contain at least two isocyanate groupsper molecule, and the derivatives of these diisocyanates which are knownin the art and which contain biuret, allophanate, urethane and/orisocyanurate groups, as well as mixtures of these polyisocyanates.Isomers or mixtures of isomers of organic diisocyanates are preferablyused.

The polyisocyanate component which is used for the production of theprepolymer can also contain small proportions of polyisocyanates ofhigher functionality.

The prepolymer is advantageously produced in the presence of catalysts,such as, organotin compounds or tertiary amines for example.

The polyurethane resins are converted into the aqueous phase asdescribed for the polyester resins, namely by neutralization of thepolyurethane resin which contains acid groups with a basic neutralizingagent. Examples of basic neutralizing agents include those which weredescribed above for the neutralization of the polyester resins.

Crosslinking of the coating composition according to the invention ispreferably effected during stoving, by reaction with a crosslinkingcomponent. Crosslinking components are familiar to one skilled in theart. Examples include amino plast resins, particularlymelamine-formaldehyde resins; phenoplast resins, blocked polyisocyanatesor transesterification crosslinking agents such as polyesters orpolyurethane esters comprising hydroxyalkyl ester groups, derivatives ofacetoacetic acid or malonic acid alkyl esters,tris(alkoxycarbonylamino)triazine derivatives, and mixtures of thesecrosslinking components, which can give rise to highly crosslinkedcoatings with or without the action of catalysts. Blockedpolyisocyanates are preferred.

These blocked polyisocyanates contain on average more than oneisocyanate group, preferably, at least two isocyanate groups permolecule. They should be stable on storage in the aqueous phase at a pHcorresponding to neutral to slightly basic conditions, should split offunder the action of heat at about 100° C. to 200° C. and shouldcrosslink with the reactive hydroxyl and/or carboxyl groups which arepresent in the resin system.

Blocked polyisocyanate are obtained by the reaction of polyisocyanateswith mono-functional compounds comprising active hydrogen.

Suitable polyisocyanates which can be used individually or in admixturein blocked form as crosslinking agents comprise any organic di-and/orpolyisocyanates which contain aliphatically, cycloaliphatically,araliphatically and/or aromatically bonded free isocyanate groups.

The preferred polyisocyanates are those which contain about 3 to 36,most preferably 8 to 15, carbon atoms. Examples of suitablediisocyanates include toluene diisocyanate, diphenylmethane diisocyanateand particularly hexamethylene diisocyanate, tetramethylxylylenediisocyanate, isophorone diisocyanate, dicyclo-hexylmethane diisocyanateand cyclohexane diisocyanate.

Examples of diisocyanates which are particularly suitable include“lacquer polyisocyanates” based on hexamethylene diisocyanate,isophorone diisocyanate and/or dicyclohexylmethane, wherein these alsoinclude the derivatives of these diisocyanates which are known in theart and which contain biuret, urethane, uretdione and/or isocyanurategroups.

Mono-functional compounds comprising active hydrogen which can be usedfor the blocking of polyisocyanates are commonly available. Example ofcompounds which can be used include acidic CH compounds such asacetylacetone; acidic CH esters, such as, acetoacetic acid ester ordialkyl malonates; (cyclo)aliphatic alcohols such as, n-butanol,2-ethylhexanol or, cyclohexanone; glycol ethers, such as, butyl glycolor butyl diglycol; phenols, such as, cresol or tert.-butyl phenol;diamino alcohols, such as, dimethylaminoethanol; oximes, such as,butanone oxime, acetone oxime or cyclohexanone oxime; lactams, such as,ε-caprolactam or pyrrolidone-2; imides; hydroxyalkyl esters; hydroxamicacids and esters thereof; and pyrazoles.

The polyisocyanates can be blocked intramolecularly with identical ordifferent blocking agents. Mixtures of identical or different blockedpolyisocyanates can also be used.

The melamine-formaldehyde resins crosslink with the hydroxyl groups ofthe polyester resin with the formation of ether groups. Examples ofcrosslinking agents such as, these include triazines, such as, melamineor benzoguanamine which are condensed with aldehydes, particularlyformaldehyde, by known industrial methods in the presence of alcoholssuch as methanol, ethanol, propanol, butanol or hexanol. These arepreferably methanol-etherified melamine resins such as Cymel®325,Cymel®327, Cymel®350, Cymel®370 or Maprenal® MF 927; butanol- orisobutanol-etherified melamine resins such as Setamin US 138 orMaprenal® MF 610 for example; and mixed-etherified melamine resins, aswell as hexamethylol melamine resins in particular, such as Cymel®301 orCymel®303.

On account of the low content of organic solvent in component (A), it isadvisable to add a customary biocidal component, such as formaldehydedeposition products, phenolic compounds, organic sulphur compounds oroxidising agents, to prevent infestation by microorganisms such asbacteria, yeast, algae or fungi.

Commercially available anionically and/or non-ionically stabilizedemulsifiers can also be used, in amounts up to 3% by weight calculatedwith respect to the solid resin, for the production of component (A).Customary lacquer adjuvant substances and additives can also be added inthe usual amounts during the production of components (A). Examplesthereof include optical brighteners such as derivatives of stilbene,coumarin, 1,3-diphenylpyrazoline, naphthalimide, benzoxazole andthiophene benzoxazole, customary catalysts such as those which are knownto one skilled in the art for the crosslinking systems concerned; andethoxylated or propoxylated derivatives of substituted phenols or fattyalcohols comprising more than 10 C atoms as film-forming agents.

Aqueous, pigmented component (B) contains one or more paste resins,pigments and/or extenders, neutralizing agents and water, advisedlycontains a biocidal component, and also optionally contains crosslinkingagents and/or customary lacquer additives and adjuvant substances suchas those described for component (A) for example.

Film-forming agents can be added, for example, in amounts of up to 10%by weight with respect to the solids content of components (A) and/or(B).

They can be added to components (A) and/or (B) or to aqueous components(A) and/or (B) or to the electro-dip lacquer coating bath which iscapable of forming a coating. Film-forming agents are preferably addedto the binder vehicles of components (A) and/or (B) before theconversion thereof into an aqueous dispersion.

Suitable paste resins include polyester resins, polyurethane resins,polyacrylate resins and amino plastic resins, such as those describedfor component (A). Polyester urethane resins are preferred.

Urethanized, oil-free polyesters which contain OH groups, and which havean acid number of 10 to 50 and a number average molecular weight (Mn) of2000 to 20,000, constitute one example of a particularly preferredembodiment. Polyester urethane resins of this type are obtained, forexample, by the reaction of one or more polyester polyols, which arefree from carboxyl groups and which have an OH number of 35 to 200 and anumber average molecular weight of 500 to 5000, comprising 2 to 30% byweight with respect to the polyester polyol of low molecular weightdiols with a molecular weight of 60 to 350, wherein a portion of the lowmolecular weight diols contains at least one acid group which is capableof forming anions, and comprising 0 to 6% by weight with respect to thepolyester polyol of low molecular weight triols with a molecular weightof 60 to 350, with one or more diisocyanates, wherein the ratio of theOH groups of the polyester polyol, diol and triol to the NCO groups ofthe diisocyanate is greater than 1.0 to 1.3. Production of the polyesterurethane resins is effected, for example, at temperatures of 20 to 150°C., preferably 45 to 90° C., optionally with the addition of catalystssuch as organotin compounds or tertiary amines. Addition polymerizationis effected in the melt or after dilution with dry solvents which do notreact with isocyanate groups, after rapid mixing of the components withintensive stirring. Polymerization proceeds until practically all theisocyanate groups have reacted. The reaction can also be carried out insteps. A different procedure can also be used when step-wise productionis employed. For example, the diol which forms anionic groups, such asdimethyolpropionic acid, can first be reacted with one or morediisocyanates in an organic solvent which does not react with isocyanategroups, whereupon it is reacted further with a polyester and a lowmolecular weight diol and/or triol which is free from anionic groups.The addition polymerization can optionally be stopped at a desired stateof reaction by mono-functional additives, such as, butanone oxime,dibutylamine or an alcoholic solvent. The function of the solvent, whichdoes not react with the isocyanate groups, is to maintain the reactantsin a liquid state and to facilitate better temperature control duringthe reaction. Examples of suitable solvents include dimethylformamide,dimethylacetamide, 1-methyl-2-pyrrolidone, acetonitrile,tetrahydrofuran, dioxane, esters, such as, ethyl acetate and alsoketones such as acetone, completely etherified mono- or diglycols ofethylene glycol or propylene glycol, as well as ketones which aresubstituted with methoxy groups.

Before the polyester urethane resin is converted into the aqueous phase,the aforementioned biocides, crosslinking agents and/or customarylacquer additives and adjuvant substances are optionally added thereto.This is followed by conversion into the aqueous phase as described forcomponent (A).

Customary pigments, extenders, corrosion inhibitors and lacquer adjuvantsubstances can be used for the pigmentation of aqueous component (B) aslong as these additives do not undergo unwanted reactions with water inthe slightly basic to neutral pH range and do not drag in anywater-soluble extraneous ions which cause problems.

Examples of suitable pigments include inorganic pigments, e.g. whitepigments, such as, titanium dioxide, zinc sulphide, lithopone, leadcarbonate, lead sulphate, tin oxide or antimony oxide; colouredinorganic pigments such as chrome yellow, nickel titanium yellow, chromeorange, molybdenum red, iron oxide red, mineral violet, ultramarineviolet, ultramarine blue, cobalt blue, chromium oxide green or ironoxide black; colored organic pigments, such as, toluidine red, litholred, perylene red, thioindigo red, quinacridone red, quinacridoneviolet, phthalocyanine blue, indanthrene blue or phthalocyanine green,carbon black, graphite, corrosion inhibitors, such as, zinc chromate,strontium chromate, zinc phosphate, lead silicochromate, bariummetaborate and zinc borate.

Effect pigments such as aluminium bronzes, pearl gloss pigments orinterference pigments can also be used. Examples of extenders which canbe used include calcium carbonate, silica, aluminium silicates,magnesium silicate, mica, barium sulphate, aluminium hydroxide andhydrated silicas.

Customary adjuvant substances such as anti-foaming agents, dispersingaids and agents for controlling the rheology can also be added toaqueous, pigmented component (B).

Aqueous, pigmented component (B) is produced in the customary mannerknown to one skilled in the art by dispersing the pigments and adjuvantsubstances in the apaste resin. The composition of the constituents toachieve optimum dispersion is determined separately for each dispersinginstallation. Examples of suitable dispersing installations includeagitator disc units, triple roller mills, ball mills or preferably sandor bead mills.

Components (A) and (B) are used for coating in a mixture ratio whichranges from 1:1 to 4:1 with respect to the weight of the aqueouscomponents concerned.

If compensation by replenishment is effected in an ADL bath which is inoperation, the two components are mixed in the aforementioned mixtureratio with the bath material. The two components can be added to thebath material simultaneously or in succession for this purpose. Thecomponents are preferably pre-mixed with part of the bath material in acustomary mixer unit. A mixer unit of this type may for example be astirred vessel, a static mixer or a rotor/stator mixer. Components (A)and (B) can also be mixed beforehand in the desired mixture ratio andused as a single-component material for compensation by replenishment.

When an ADL bath is first prepared, component (A) is treated withadditional neutralizing agent in order to obtain the desired MEQ valueof the ADL bath and is optionally pre-diluted with water. Thereafter,component (B) is added in the manner described above and the mixture isadjusted to the desired solids content for coating.

In another variant of the method, the necessary amount of water is firstplaced in the tank with the neutralizing agent and components (A) and(B) are added in the manner described above.

In continuous operation, the ADL bath has a solids content of 8 to 25%by weight, preferably 10 to 15% by weight, an MEQ value of 50 to 90,preferably 60 to 70, and a content of organic solvents which is lessthan 0.3% by weight.

Deposition is effected by applying a DC voltage of 50 to 500 volts for acoating time of 0.5 to 5 minutes, at an ADL bath temperature of 18 to35° C.

The coating material is suitable for the coating of workpieces whichhave an electrically conducting surface, and is particularly suitablefor the priming and single-coat lacquering of domestic and electricalappliances, steel furniture, building components, building andagricultural machines, automobile bodies and automobile accessories.

The following examples illustrate the invention. All parts andpercentages are on a weight basis unless otherwise indicated,

EXAMPLES

1. Production of an Aqueous, Pigment-free Binder Vehicle Component Freefrom Crosslinking Agents (A1)

A mixture of 2.55 parts by weight dimethylethanolamine (50%) and 3 partsby weight of deionized water was added to 57.65 parts by weight of apolyester resin with an acid number of 49 and a hydroxyl number of 60(produced from 26.17 parts by weight neopentyl glycol, 5.43 parts byweight trimethyolpropane, 10.83 parts by weight isophthalic acid, 21.45parts by weight isodecanol and 36.12 parts by weight trimelliticanhydride) in a reaction vessel fitted with a stirrer, thermometer andreflux condenser. The batch was stirred at 100° C. for 10 minutes untilhomogeneous, and then 0.15 parts by weight of a commercially availablebiocide were likewise stirred in for 10 minutes until homogeneous. 36.65parts by weight of deionised water were added, with stirring. Themixture was stirred for 90 minutes at 80° C. and was subsequently cooledrapidly to 25° C.

Characteristic properties: solids content (30 minutes at 180° C.): 57%MEQ amine: 29 milliequivalents amine/100 g solid resin solvent content:<0.1%

2. Production of an Aqueous Pigment-free Binder Vehicle Containing aCrosslinking Agent (A2)

0.12 parts by weight of a commercially available non-ionic emulsifierwere stirred into 47.75 parts by weight of a polyester resin with anacid number of 49 and a hydroxyl number of 60 (produced from 26.17 partsby weight neopentyl glycol, 5.43 parts by weight trimethyolpropane,10.83 parts by weight isophthalic acid, 21.45 parts by weight isodecanoland 36.12 parts by weight trimellitic anhydride) in a reaction vesselfitted with a stirrer, thermometer and reflux condenser. 8.03 parts byweight of a solvent-free crosslinking agent (an isocyanurate ofhexamethylene diisocyanate, blocked with butanone oxime) were heatedbeforehand to 70 to 80° C., added to the mixture and stirred in for 15minutes until homogeneous. A mixture of 1.38 parts by weightdiisopropanolamine (50%), 0.7 parts by weight aqueous ammonia and 2.60parts by weight of deionized water was subsequently added and stirred infor 10 minutes until homogeneous.

Thereafter, 0.15 parts by weight of a commercially available biocidewere added and stirred in for 10 minutes until homogeneous. 39.27 partsby weight of deionised water were added, with stirring. The mixture wasstirred for 90 minutes at 80° C. and was subsequently cooled rapidly to25° C.

Characteristic properties: solids content (30 minutes at 180° C.): 53%MEQ amine: 32 milliequivalents amine/100 g solid resin solvent content:<0.1%

3. Production of an Aqueous, pigment-free Binder Vehicle Componentcontaining a Crosslinking Agent (A3)

9.40 kg of a melamine resin of the hexamethylol-melamine resin type wereadded to 90.60 kg of aqueous binder vehicle component (A1) in adissolver mixer with stirring, and were stirred for 30 minutes at 40° C.

solids content (30 minutes at 180° C.): 60.8% MEQ amine: 24.6milliequivalents amine/100 g solid resin

4. Production of a Solvent-free Paste Resin

453.5 g of a linear polyester of adipic acid and hexanediol with ahydroxyl number of 110 g, together with 37.1 g dimethylolpropionic acid,were dissolved in 134 g acetone at 50° C. in a reaction vessel fittedwith an internal thermometer and reflux condenser. 159.5 isophoronediisocyanate were added in such a way that the temperature of reactiondid not exceed 70° C. The temperature of reaction was maintained untilan NCO number of about 0.5% and a viscosity, as measured in a 60%solution in acetone, of about 1200 mPa.s were reached. Thereafter, 10 gbutyl glycol were added in order to deactivate the remaining NCO groups.The batch was subsequently neutralized with 30.0 g of a 50%dimethylethanolamine solution and an aqueous dispersion was producedwith 1450 g water. The acetone was removed from the reaction mixture bydistillation so that a solvent-free, aqueous polyurethane dispersion wasobtained.

Characteristic properties: solids content (30 minutes at 150° C.): 30.1%acid number: 24.1 mg KOH/g MEQ amine: 26 milliequivalents amine/100 gsolid resin

5. Production of an Aqueous, Pigmented Component (B1)

In order to produce 100 kg of pigmented component (B), 56.85 kg of thepaste resin were placed in a dissolver-mixer, and 21.20 kg coarse carbonblack and 2.12 kg of a furnace black, as well as 19.83 aluminiumhydrosilicate, were sprinkled in with stirring. The product for grindingwhich was thus produced was stirred for 15 minutes at 40° C. After aperiod of swelling of 12 hours, the material for grinding was dispersedin a coball mill under predetermined conditions.

solids content (30 minutes at 180° C.): 60.2% MEQ amine: 7.1milliequivalents amine/100 g solid resin

6. Production of an Aqueous, Pigmented Component (B2)

In order to produce 100 kg of pigmented component (B2), 42.00 kg of thepaste resin were placed in a dissolver-mixer and 41.70 kg titaniumdioxide, 7.00 kg aluminium hydrosilicate, 7.00 kg of post-treatedaluminium hydrosilicate, 1.80 kg silica and 0.50 kg of a polybutylenewere sprinkled in with stirring in the sequence given. The material forgrinding which was thus prepared was stirred for 20 minutes at 50 to 60°C. and was subsequently dispersed in a coball mill under predeterminedconditions.

solids content (30 minutes at 180° C.): 70.1% MEQ amine: 4.5milliequivalents amine/100 g solid resin

7. Production of a Black Electro-dip Lacquer Coating Bath

pigment-free aqueous binder vehicle component containing a crosslinkingagent (A3)

aqueous pigmented component (B1)

mixture ratio of component A3: component B1=3.5:1

1669.65 g of deionized water were first placed in a vessel and 7.35 g ofa neutralizing agent (100% dimethylethanolamine) were added. 252 g ofpigment-free aqueous binder vehicle component (A3) were subsequentlyadded slowly, with stirring or rotation. After homogenising for 30minutes, 71 g of aqueous pigmented component (B1) were added withstirring or circulation. After a period of homogenizing of about 1 hour,the electro-dip bath was ready to be used for coating.

Bath Properties:

pH: 8.6 conductivity: 1234 μS/cm solids content (30 minutes at 180° C.):9.8% MEQ amine: 62.9 milliequivalents amine/100 g solids

8. Production of a Grey Electro-dip Lacquer Coating Bath

pigment-free aqueous binder vehicle component containing a crosslinkingagent 25 (A2)

aqueous pigmented component (2)

mixture ratio of component A3: component B1=2.0:1

1632 g of deionised water were first placed in a vessel and 14.6 g of aneutralizing agent (50% diisopropanolamine) were added. 237.4 g ofpigment-free aqueous binder vehicle component (A2) were subsequentlyadded slowly, with stirring or circulation. After homogenizing for 30minutes, 116 g of aqueous pigmented component (B2) were added withstirring or circulation. After a period of homogenization of about 1hour, the electro-dip bath was ready to be used for coating.

Bath properties:

pH: 8.1 conductivity: 1094 μS/cm solids content (30 minutes at 180° C.):10.4% MEQ amine: 47.7 milliequivalents amine/100 g solids

What is claimed is:
 1. A method for anodic electro-dip lacquer coating,wherein coating medium which is consumed in an anodic electro-dip bathis compensated for by an under-neutralized anodic replenishmentmaterial, where in the replenishment material comprises: A) apigment-free aqueous binder vehicle component with a solid content of 40to 70% by weight, based on the weight of the vehicle component, an MEQvalue of 15 to 40 and a content of organic solvent of 0.5% by weightbased on the weight of the vehicle component, and B) apigment-containing aqueous paste resin component with a solid content of60 of 75% by weight, based on the weight of the resin component, an MEQvalue of 5 to 15 and a content of organic solvent of 1.0% by weight,based on the weight of the resin component, wherein A) and B) arepresent in a ratio by weight of 1:1 to 4:1 and the mixture of A) and B)has a solid content of 45 to 73% by weight based on the weight of themixture of A) and B), and an MEQ value which is 50 to 70% lower than theMEQ value of the electro-dip bath.
 2. A method according to claim 1,wherein component A) and/or component B) contain one or more customarybiocidal agents.
 3. A method according to claims 1 or 2, whereincomponent A) contains one or more film-forming binder vehicles,emulsifiers, and/or Customary lacquer adjuvant substances andoptionally, contains one or more crosslinking agents.
 4. A methodaccording to claim 3, wherein component B) contains one or more pasteresins, pigments and/or extenders and/or customary lacquer adjuvantsubstances, and optionally, contains one or more crosslinking agents. 5.A method according to claim 1, wherein it is carried out for the coatingof industrial products or motor vehicle bodies or parts thereof.
 6. Amethod according to claim 1, wherein it is carried out withoutelectrodialysis of the electro-dip lacquer bath.